This invention relates to treatment and disposal of waste liquid slurries containing a dissolved metal and is of particular utility in "back-end" metallization processing of integrated circuit semiconductor devices having copper-based contacts, vias, interlevel metallization, and device interconnection routing.
Metal films are utilized in semiconductor manufacturing technology to form electrically conductive contacts to active as well as passive device regions or components formed in or on a semiconductor substrate, as well as for filling via holes, interlevel metallization, and interconnection routing patterns for wiring together the components and/or regions. Metals employed for such purposes include titanium, tantalum, aluminum, nickel, cobalt, silver, gold, copper, and their alloys. Of these, copper and copper-based alloys are particularly attractive for use in VLSI and ULSI multilevel metallization systems employed for. "back-end" processing of semiconductor wafers. Copper has a very low resistivity, i.e., even lower than that of aluminum, and a significantly higher electromigration resistance. Moreover, copper and its alloys enjoy a considerable cost advantage over silver and gold and their alloys.
With reference to FIG. 1, schematically shown therein in cross-sectional view is a process of particular utility in the "back-end" metallization of integrated circuit devices, and useful in the manufacture of various electrical components, circuit boards, etc., which process employs "damascene" (or in-laid) technology to form recessed metallization patterns and/or contacts. Illustratively, substrate 1 comprises a semiconductor wafer, typically of monocrystalline silicon, comprising at least one active device region formed therein or thereon. In an initial step, the desired conductor pattern is defined as a plurality of recesses 2 such as grooves, trenches, holes, etc. in a dielectric layer 3 formed over the semiconductor substrate. A subsequent step comprises deposition of a suitably conductive metal layer 5 (e.g., copper or a copper alloy) filling the recesses 2. Typically, in order to ensure complete filling of the recesses, the metal layer 5 is deposited as a blanket layer of excess thickness t so as to overfill the recesses and cover the upper surface 4 of the dielectric layer 3. Although the upper surface 6 is shown in the figure as planar for illustrative simplicity, in practice it is highly non-planar as a result of the uneven substrate topography and the characteristics of its method of deposition.
In the next step according to damascene technology, the entire thickness t of the metal layer 5 over the surface of the dielectric layer 3 is removed by a planarization process, typically chemical-mechanical polishing ("CMP"), leaving metal portions 5' in the recesses 2 with their exposed surfaces 7 substantially coplanar with the surface 4 of the dielectric layer 3. Such damascene process forms in-laid conductors in a dielectric layer while advantageously avoiding problems associated with other types of processes, e.g., metal etching and dielectric gap filling.
In a typical CMP method employing conventional apparatus, the semiconductor wafer 1 is rotated against a rotating polishing pad while an abrasive and chemically reactive solution/slurry is supplied to the rotating pad. Other CMP apparatus may utilize an oscillating or continuous belt pad.
Slurries used for CMP of silicon semiconductor wafers can be divided into three categories, depending upon their intended use: silicon polish slurries, oxide polish slurries, and metal polish slurries. The silicon polish slurries are formulated for polishing and planarizing bare silicon wafers, whereas the oxide polish slurries are designed for polishing and planarizing a dielectric layer on a wafer, typically a layer of a silicon oxide. The metal polish slurries, which are utilized in and form the subject of the process according to the present invention, are employed for polishing and planarizing a conductive metal-containing layer on a semiconductor wafer.
As described above, the conductive metal-containing layer is typically deposited on a dielectric layer and can comprise tungsten, titanium, aluminum, nickel, cobalt, copper, silver, gold, and alloys thereof. Commonly employed CMP metal polishes include very small particles of an abrasive, such as silica, alumina, or ceria, having a diameter of about 50-1,000 nm, suspended in a water-based liquid vehicle. The proportion of the particles in the slurry depends upon the particular slurry used and typically is in the range of about 1-5% by weight. The pH of a metals polish slurry may vary from slightly acidic (e.g., about 5.0) to approximately neutral (e.g., about 7.5), depending upon the particular formulation used. In the case of the slightly acidic formulations, the pH is optionally controlled by the addition of an organic acid such as acetic or citric acid. In addition, the slurry may contain one or more oxidizing agents for solubilizing the conductive metal-containing material and, thus, assist in its removal. Typical oxidizers include hydrogen peroxide, potassium ferricyanide, ferric nitrate, and/or mixtures thereof. Additional details concerning compositions of metals polishing slurries for CMP processing of semiconductor wafers, as well as process parameters, are described in U.S. Pat. No. 5,340,370, the entire disclosure of which is incorporated herein by reference.
During CMP, the abrasive action of the slurry particles on the metallization layer(s) and pattern(s) on the wafer surface results in the removal of very small particles of metal (i.e., on the order of about 0.2 um), which particles are rapidly solubilized (i.e., dissolved) by the oxidizing and other proprietary agents contained in the CMP slurry. As a result, used or spent CMP, even when diluted with rinse water, contains a significant concentration of dissolved metal.
In some CMP systems, spent slurry and rinse water are not segregated, with both being directed down a waste drain. The volume of rinse water used is typically more than thirty times and as much as one hundred times the volume of spent slurry. In a typical semiconductor manufacturing plant, from about 10 to about 100 gpm of fabrication polishing wastes (i.e., spent slurry+waste water) are discharged to a waste drain. The large amount of chemical consumption due to passage of such large waste volumes adds considerably to the adverse environmental impact of wafer manufacture. An important consideration also is the toxicity of several of the metals which may be dissolved in the spent slurry as a result of CMP of particular metallization systems, which toxicity may impose severe environmental constraints on discharge of CMP wastes. In addition to the above, a significant economic consideration of such CMP waste disposal is the loss of expensive metals, e.g., silver and gold, dissolved in the CMP waste stream.
Moreover, although CMP slurries are expensive, the risk of damaging a wafer whose value is in the tens of thousands of dollars militates against any possible use of recycled slurry as a means of cost savings. As a practical matter, the risk of wafer damage from use of recycled slurry cannot be greater than the risk of damage from the use of fresh slurry. Consequently, recycling of spent CMP slurry is not commonly practiced in the semiconductor manufacturing industry.
In another conventional approach to disposal of CMP wastes, spent CMP slurries are treated, as by filtering, to separate the solids (polishing aggregates) therefrom prior to discharge of the liquid filtrate into a drain. The separated solids are then supplied to a filter press for compressing the solids into a filter cake ("sludge") for off-site disposal. However, inasmuch as such sludge is, in many areas, considered a hazardous waste because of the process that generated it, handling of such off-site disposal is not desirable under many circumstances.
As an optional adjunct to the above process, in some instances the filtrate obtained from the solids separation is subjected to processing, typically ion exchange, for removal of the dissolved metal prior to discharge to the waste drain. As a further option, the dissolved metal may be recovered from the filtrate in a solid form for re-use.
Thus, there exists a need for a method and system for treating spent or waste CMP slurries containing at least one dissolved metal, which does not suffer from the problems and drawbacks of the prior art, i.e., discharge of large amounts of untreated process waste liquids into a waste drain, discharge of toxic or otherwise environmentally hazardous metals into the waste stream, generation and problems associated with disposal of hazardous solid waste sludge, and loss of expensive metals used in "back-end" metallization processing of semiconductor wafers.
Moreover, there exists a need for an improved method and system for treating CMP waste slurries which is compatible with existing CMP methodology and fully satisfies environmental requirements and standards for disposal of both the solid and liquid components of CMP processing wastes.